I use a Tascam BT-1 and I couldn't live without it. Ok, I could but it would be harder. However, I almost never use it at 1/2 speed. By the time you go to 1/2 speed you cannot hear the notes well enough.

But, there's more! It can also boost the bass and change the pitch. It can cut the bass too, but I find it hard to play along to songs with no bass drum

Plus it has all sorts of cheesy effects you can play with when you are bored.

.. and I LOVE it, INCREDIBLY useful, particularly for playing along with a CD track that's too fast for me to play accurately up to speed; you can decrease the speed by percentage, without affecting the pitch...
... of course, the more you slow it down, the more it degrades the sound, but I find it to be no problem down to about 60% of original tempo.

...btw, I'm still using the free Trial version, but I'm gonna buy it for sure.....inexpensive and Hella useful practice tool